Reactions of N-Monosubstituted Amidines with AlMe₃ and AlMeCl₂: Formation of Fused Triazaalane Heterocycles

Reactions of N-monosubstituted amidines of the formula HNC­(R)–NH­(R′) (R = Ph, 4-tert-butylphenyl, Me; R′ = 2,6-diisopropylphenyl, Ph) with AlMe3 and AlMeCl2 are reported. All the N-(Dipp)­amidines (Dipp = 2,6-diisopropylphenyl) react with AlMe3 in a 1:1 ratio to yield tetrameric aluminum amidinates (1, 2, and 3) in good yields. In these compounds, the amidinate ligand chelates to Al while bridging to another Al. However, when N-phenylamidines are employed, tetracyclic (4–9) and pentacyclic (10) Al–N–C heterocycles are formed. In the case of N-phenylbenzamidine, formation of a hexacyclic Al–N–C heterocycle (11) is observed when a slight excess of AlMe3 (1:1.2 equiv) is used. In these compounds, the amidinate ligand coordinates to Al atoms in a bridging fashion. Further, N-(Dipp)­acetamidine and N-(Dipp)­benzamidine are also treated with AlMeCl2. Whereas N-(Dipp)­acetamidine affords an ionic 15-membered aluminum amidinate chain (12), N-(Dipp)­benzamidine gives a bicyclic heterocycle (13) and the AlCl3 adduct of N-(Dipp)­benzamidine (14). However, from a reaction between N-phenylbenzamidine and AlMeCl2, only the AlCl3 adduct, 15, was isolated. Compounds 3, 4, 6, 8, and 10–15 have been structurally characterized.